1. Field of the Invention
This invention relates to tunnel diodes and the manufacture thereof, and more particularly to tunnel diode detectors particularly useful in microwave frequency circuits.
2. Description of the Prior Art
Early tunnel diodes were produced by a planar process in which the junction was created by an alloy process. An article entitled "Design Principles and Construction of Planar Ge Esaki Diodes" by R. E. Davis and G. Gibbons, which appeared in Solid-State Electronics, 1967, volume 10 pages 461-472 describes an alloyed junction tunnel diode and processes utilized to construct such a diode. The authors note that they prefer the alloy process for forming the junction, as contrasted to using diffusion, or high temperature vapor deposition of N+ germanium doped with phosphorus, which is grown on P+ germanium substrates.
An example of a typical process for producing an alloy junction tunnel diode of the type described in the Davis et al. article is illustrated in FIGS. 1a through 1f. Referring to FIG. 1a, the process for producing an alloyed junction tunnel diode begins with providing a germanium substrate 1 which is doped with a P type dopant, such as for example, gallium, to a doping concentration of about 5.times.10.sup.19 atoms/cm.sup.3. A layer of silicon dioxide 2 is formed on surface 3 of substrate 1 by for example, chemical vapor deposition to cover surface 3 to a thickness of approximately 10,000 .ANG. (FIG. 1b). Next, surface 5 of layer 2 is patterned, using conventional photolithography techniques, and opening 4 is etched down to surface 3 using wet or dry etching techniques (FIG. 1c). Opening 4 is used to define the junction between the to be applied alloy and surface 3 of germanium substrate 1. Following the definition of opening 4, a photoresist (not shown) is applied to surfaces 5 and 3 of FIG. 1c and patterned to define the desired configuration of the alloy to be applied. FIG. 1d illustrates the structure which results from the application of alloy 6, which may be for example Sn-As or Sn-Sb, which is typically applied by the process of metal evaporation, followed by the removal of the photoresist. After deposition of alloy 6 the metal alloy is alloyed with the Ge semiconductor at an elevated temperature. Top contact 7 is applied to exposed surface 8 of alloy 6 by a process of for example, gold plating, to a thickness of 3-6 microns (FIG. 1 e). To complete the tunnel diode structure, contact to substrate 1 is provided by the application to surface 9 of back contact 10, which may consist of, for example, gold material, which may be applied by the process of evaporation to a thickness of, for example, 3,000 .ANG..
One of the difficulties in producing desirable tunnel diodes of the type utilizing an alloy junction, as described above, arises because of the nonuniformity of the junction between the doped alloy metal and the oppositely doped semiconductor substrate. To illustrate this nonuniformity problem, FIG. 1g is provided to show a highly enlarged representation of the area in FIG. 1d which is encircled and indicated by reference character g. In FIG. 1g the jagged line separating substrate 1 and alloy 6 illustrates the nonuniform composition of N+/P+ junction which is the result of the alloying and recrystallizing between alloy 6 and substrate 1. This nonuniformity results in the problems of non-uniformity between devices produced by the process, thermal instability, and low power handling capability.
Another prior art technique which has been reported for the production of tunnel diodes involves the utilization of vapor growth of doped germanium on either a P-doped germanium substrate or a P doped gallium arsenide substrate. For example, this type of device is described in an article by J.C. Marinace entitled "Tunnel Diodes by Vapor Growth of Ge on Ge and on GaAs", reported in the IBM Journal, July 1960, pages 280-282.
In the category of devices having the construction of Ge on Ge, the article describes two processes for producing this combination. The first process is described as the "closed-tube process", and the second the "open-tube process". In the closed-tube process, the substrate is comprised of gallium doped germanium onto which a layer of germanium which is phosphorus doped is grown from a monocrystalline germanium source which includes a phosphorus doping. In the open-tube process, the author describes experimental junctions produced on a gallium doped germanium substrate by the vapor deposition from either arsenic- or phosphorus-doped source germanium. Tunnel diodes produced with the germanium or germanium combination described in the article do not lend themselves well to use in microwave frequencies because it is not possible to integrate other components on the germanium substrate since germanium cannot be made semi-insulating.
In the Marinace article the second combination of vapor grown tunnel diodes involves the utilization of a GaAs substrate, which is zinc doped (providing a highly P-doped substrate), onto which P-doped germanium is vapor deposited in the "open-tube"process. Tunnel diodes produced by vapor deposition of Ge on a GaAs substrate are not totally satisfactory because it is difficult to grow an abrupt, degenerately doped P+/N+ junction. Without the desired abrupt junction, the device suffers from the disadvantages of: (i) lower peak to valley current ratios; and (ii) less microwave sensitivity.
Another tunnel diode, or Esaki diode, is described in Japanese Pat. No. SHO 56-31903, by Yoshito Amemiya entitled "An Esaki Diode", issued July 24, 1981 and assigned to Nippon Telegraph and Telephone Corporation. This patent discusses the optimization of doping concentrations in Esaki diodes to maximize the switching speed of the diode. In the description of the invention, the inventor proposes that the preferred composition for the N-type semiconductor region (to achieve fast switching) be In.sub.x Ga.sub.(1-x) As (provided that 0.1.ltoreq..times..ltoreq.0.6), InAs.sub.y P.sub.(1-y) (provided that 0.1.ltoreq.y.ltoreq.0.6), or In.sub.z Ga.sub.(1-z) Sb (provided that 0.1.ltoreq.z.ltoreq.0.95). The emphasis in the article, as mentioned above, is on maximizing switching speed and this is addressed in terms of selecting dopant concentration ratios for the N-type semiconductor region. No particular attention is directed to the P-type semiconductor region. The article does not mention the implementation of a tunnel diode on a semi-insulating substrate, nor does it address the characteristics desired of a tunnel diode to be used as a microwave detector.